Background

As of December 2003, 45 states had issued fish advisories for mercury covering more than 13,000,000 lake acres and over 750,000 river miles. Atmospheric deposition of mercury is a primary route of transport of mercury to water. Mercury air emissions from coal-fired power plants, waste incinerators, mercury cell chlorine manufacturing facilities, and other sources can be transported long distances before ultimately depositing on watersheds and waterbodies. Much of the mercury deposited within state boundaries is generated from emission sources outside the state. The Mercury Maps approach can be used to coordinate EPA's efforts to address mercury contamination in water quality, wastewater permitting, and air emission programs.

Environmental and public health benefits

The Mercury Maps approach could be used to help evaluate the benefits of anticipated reductions in air deposition. As one component of an overall benefits analysis, this method could be used to relate reductions in air deposition rates to reductions in fish tissue concentrations, by watershed, nationwide. The Mercury Maps approach could also be used to perform individual Total Maximum Daily Load (TMDL) analyses and/or to perform TMDL analyses for multiple watersheds at a time. States could use this approach to develop state-wide strategies for addressing the fish mercury contamination problem. While this analysis addresses only effects on inland freshwater fish and is based on the new methylmercury criterion, most state standards are water column concentration-based. States would need to adopt fish tissue based target levels in order to benefit from this approach. That is, the target levels would need to be either a single fish tissue criterion, applicable to all waters in the state, or a single fish tissue criterion converted to site-specific water column criteria, based on bioaccumulation factors measured for each waterbody. In addition, most state fish consumption advisories are set at target levels that are about half the criterion concentration.

Technical approach

Mercury Maps is a peer-reviewed geographic information system (GIS) based analysis with national data coverages for watersheds, fish tissue concentrations, and non-air-deposition source locations. The model used in this project is a reduced form of the watershed and aquatic ecosystem effects models used in the 1997 Mercury Study Report to Congress. The equations from these models have been reduced to a single equation and applied directly in the GIS.

Mercury Maps is a tool that relates changes in mercury air deposition rates to changes in mercury fish tissue concentrations, on a national scale. Mercury Maps suggests that changes in steady-state concentrations of mercury in fish will be proportional to changes in mercury inputs from atmospheric deposition if air deposition is the only significant source of mercury to a water body; and if the physical, chemical, and biological characteristics of the ecosystem remain constant over time. This model is best applied to ecosystems where atmospheric deposition is the principal source of Hg to a water body and assumes that the physical, chemical, and biological characteristics of the ecosystem remain constant over time. Estimates of percent air deposition reductions, by watershed, as generated from a regional air deposition model, would be needed to predict fish concentration reductions.

Limitations

Mercury Maps has several limitations:

The Mercury Maps approach is based on the assumption of a linear, steady-state relationship between concentrations of methylmercury in fish and present day air deposition mercury inputs. This condition may not met in many waterbodies because of recent changes in mercury inputs and other environmental variables that affect mercury bioaccumulation. For example, the US has recently reduced human-caused emissions and international emissions have increased.

The requirement that environmental conditions remain constant over the time required to reach steady state inherent in the Mercury Maps methodology may not be met, particularly in systems that respond slowly to changes in mercury inputs.

Many water bodies, particularly in areas of historic gold and mercury mining in western States, contain significant nonair sources of mercury. Mercury Maps methodology cannot be applied to these waterbodies.

Finally, Mercury Maps does not provide for a calculation of the time lag between a reduction in mercury deposition and a reduction in the methylmercury concentrations in fish.

Despite the limitations of this model, EPA is unaware of any other tool for performing a national-scale assessment of the change in fish methylmercury concentrations resulting from reductions in atmospheric deposition of mercury.

* States currently use water column concentration-based mercury water quality standards and would need to adopt fissue tissue-based target levels in order to use this approach for mercury TMDLs. Additional reductions would be required to meet EPA national and most state fish advisory threshold concentrations, which are often set below the methylmercury criterion.

Note: Watersheds highlighted yellow have "significant" mercury sources other than deposition, defined as where the total estimated load from Publicly Owned Treatment Works (POTWs) and pulp and paper mills is greater than 5% of estimated waterbody delivered mercury at a typical air deposition load (10 g/km2/yr), and/or where mercury cell chlor-alkali facilities, mercury mines, or significant past producer gold mines are present. See text of report for data sources for point source dischargers and mines.

Source: National Listing of Fish and Wildlife Advisories (NLFWA) Mercury Fish Tissue Database (June 2001). The national Mercury Maps analysis is based on the 22,000 georeferenced records of fish tissue mercury concentrations in 35 states (west coast and states in the eastern two thirds of the U.S.) in the National Listing of Fish and Wildlife Advisories (NLFWA) Fish Tissue Database available at the time of the analysis (June, 2001). Substantially more fish tissue data is currently available. This national analysis screens watersheds that contain potentially significant, but unquantified, runoff and effluent loads from mercury mines, large-producer gold mines, and mercury-cell chlor-alkali facilities. Watersheds are also eliminated when the total screening level effluent load estimates for municipal wastewater treatment plants and pulp and paper mills are above five percent of the estimated waterbody-delivered air deposition load.